Affirmative action IS discrimination.
That said... who cares? The action of "discrimination" has been given a bad and undeserved reputation. We discriminate between intelligent and unintelligent people; beautiful and ugly people; black people and white people. There are valid reasons in certain situations for most forms of discrimination. Do you want unintelligent people designing aircraft? Do you want ugly people to be hired as fashion models? Should urban police officers pretend that it's mostly whites dealing drugs in black neighborhoods?
Forget fairness. Merit isn't the overriding consideration in electoral politics (polularity is), business (profitability is), or in sexual relationships (physical appearance often is), AND IT SHOULDN'T BE. The salient question isn't whether affirmative action is discrimination - it is - but whether discrimination is appropriate in THIS particular context. Consider: when discrimination has a rational basis (see above examples), no one finds it morally offensive. People usually find discrimination morally offensive only when it is intended as a slight against some group. The question, then, is whether this discrimination has a rational basis. For what it's worth, I think it is immediately apparent that the goal of student body racial diversity is a legitimate goal, and consequently, this discrimination shouldn't be considered morally offensive.